This invention relates to a process and system for making a slurry having a solids content within a qualification range. More particularly, the invention relates to a process and system that utilize a reference conductivity value indicative of when a sufficient amount of at least two chemicals are combined to provide a slurry with the desired solids content. By monitoring conductivity while the chemicals are combined and comparing the monitored conductivity value to the reference conductivity value, the process and system of the present invention are able to easily and accurately monitor and/or provide a slurry having a solids content within a desired qualification range.
Chemical mechanical polishing (CMP) has become an essential process in the production of integrated circuits. CMP is the process of planarizing a surface, typically a wafer surface, such that it becomes smoothed and flattened. Generally, the CMP process utilizes a chemical slurry in combination with a polishing pad to achieve the desired surface finish. Typically, chemical slurries useful in CMP comprise a solvent and plurality of solid, abrasive particles insoluble in the solvent. In the CMP process, the slurry is applied to the surface to be planarized and moved about the surface due to the motion of the polishing pad. In this manner, the particles in the slurry abrasively planarize the surface in a manner similar to sanding. Desirably, the CMP process is carried out at a known polishing rate to get planarized surfaces of desired thicknesses.
Inasmuch as the quality of the CMP process is determinative of the quality of the planarized surface produced thereby, variability in the CMP process is undesirable. Variability in the CMP process can be introduced as a result of variability in the quality of the CMP slurry. Thus, maintaining consistent slurry quality is crucial to maintaining of the consistency of the CMP process and thus, to maintaining consistent manufacturing output. Philipossian, et al., xe2x80x9cAn Overview of current Issues and future Trends in CMP Consumables,xe2x80x9d Proceedings of the First CMP-MIC Conference, February 1996, pp. 13-19. To this end, slurry quality is desirably monitored to decrease the possibility that slurry is a major contributor to CMP variability.
More specifically, a slurry to be used in a CMP process should have a tightly controlled solids content, i.e., weight concentration of solid particles, as even slight variations in solids content can cause dramatic variation in the rate and quality of polishing. It is further desirable that the slurry be stable, i.e., the slurry should maintain consistent polishing performance, while also resisting the tendency of the particles to settle out or agglomerate. It is additionally desirable that a chemical slurry used in a CMP process should be relatively free of contaminants, foreign particles, carbon dioxide, ionic contaminants and the like.
A variety of different instruments and/or methodologies have been proposed for monitoring slurry quality. For example, off-line measurements, such as density or specific gravity measurements, measurement of percent nonvolatile solids, or component assays, have been proposed as useful indicators of slurry quality. However, for a variety of reasons, none of these measurements are optimal indicators of slurry quality. Specifically, measurement of percent nonvolatile solids, which involves an evaporation to dryness and subsequent measure of the remaining weight of solids, requires application of high temperatures in order to provide the desired efficiency and repeatable results. However, many slurries contain dissolved solids that decompose at the high temperatures required by this procedure. For example, many tungsten slurries contain dissolved solids with low decomposition temperatures that completely decompose at the required high temperatures. As a result, this methodology often produces erroneously low values for percent nonvolatile solids in these slurries. J. P. Bare, xe2x80x9cImproved Analytical Technique for Metal CMP Slurry,xe2x80x9d Proceedings of 2nd CMP-MIC Conference, February 1997, pp. 405-408.
Additionally, component assays, i.e., assays that measure the concentration of dissolved solids (potassium iodate, ferric nitrate, etc.) in the concentrated slurry component, have also been used as indicators of slurry quality. The most common component assay is an off-line titration for the specific dissolved solid in question. However, many commercially available slurries contain proprietary ingredients. Thus, composition and concentration information for these slurries is not known and not readily available. Without this information, the use of component assays as a mechanism for monitoring slurry quality is challenging, if not completely impossible. Finally, each of the proposed off-line measurement techniques necessarily require sampling and subsequent analysis outside of the manufacturing process, and thus, do not provide a real-time indicator of slurry quality.
Thus, on-line measurements have been proposed, and are considered generally more attractive, as providing the desired real-time indication of slurry quality. For example, pH and density may be measured on-line. However, neither of these on-line approaches have proven to be an optimal indicator of slurry quality for a variety of reasons. Specifically, the measurement of pH of a chemical slurry may not be an accurate indicator of slurry quality for chemically buffered slurries. Furthermore, even in unbuffered oxide slurries, silica particles can act collectively as a buffering agent. Finally, the reliability of pH as an indicator of slurry health is adversely effected by the limited precision and/or resolution of most pH meters when used on-line. That is, standard pH probes are prone to clogging of the porous probe membrane by the small particles present in a slurry. Thus, frequent regular maintenance of these instruments is required. Furthermore, in order to ensure maximum reliability and accuracy of these instruments, frequent calibration is required.
The measurement of density and the remainder of on-line measurement techniques also fail to provide the consistent, accurate indicator of slurry quality desired in the CMP process. Specifically, although on-line measurement is attractive in that it provides a real-time indicator of slurry quality, many analytical instruments are not easily adapted to on-line sampling. Furthermore, as is the case with pH meters, other on-line instruments also require rigorous maintenance and calibration in operation, rendering them costly and inefficient methods of slurry monitoring.
Thus, there is a need for an efficient, easy-to-use system and process capable of providing a real-time indication of slurry quality.
According to the present invention, the above objectives and other objectives apparent to those skilled in the art upon reading this disclosure are attained by the present invention which is drawn to a process and system for making and/or monitoring the character of a slurry having a solids content within a qualification range. More specifically, it is an object of the present invention to provide a process and system that utilize conductivity to monitor solids content. By monitoring conductivity on-line while mixing chemical components, the process and system of the present invention provide an exceptionally accurate, reliable, easy-to-use means for monitoring and maintaining the quality of a chemical slurry.
Generally, the process involves providing a reference conductivity value corresponding to a reference slurry having certain desired characteristics, e.g., characteristics indicative of when sufficient amounts of at least two chemical components have been combined to form a slurry with the desired composition. The at least two components are combined, and, when the measured conductivity of the combined components corresponds to the reference conductivity value, the slurry is ready to use. In preferred embodiments, the conductivity is monitored as the at least two components are combined.
For example, the process of the present invention may be used to generate two-component slurries. In such slurries, one of the components, i.e., a first component, typically comprises a first solvent and the other component, i.e., a second component, comprises a second solvent and a plurality of solid particles. A reference conductivity value is provided indicative of when sufficient amounts of the first and second components have been combined to provide a slurry with a solids content within the qualification range. The first and second components are then combined until the combination, i.e., the blended slurry, has a measured conductivity value that corresponds to the reference conductivity value. In this manner, a slurry having a desired solids content is produced.
The process of the present invention is also suitable for the preparation of slurries comprising three or more chemical components. In this embodiment of the invention, both a predetermined order for combining the plurality of components, as well as a reference conductivity value for each combining step, are provided. More specifically, a predetermined order is provided in which the plurality of components are to be combined in a series of steps. Then, as each combining step is performed, the components corresponding to each step are combined until the measured conductivity corresponds to the reference conductivity value associated with the combining step. When all of the combining steps have been completed in this manner, the resultant slurry is ready to use.
In preferred embodiments, the rate of addition of the component comprising a plurality of solid particles may be varied in response to the conductivity measurement, i.e., the addition may be slowed as the measured conductivity approaches the reference conductivity value, and ceased when the measured conductivity of the blended slurry corresponds to the reference conductivity value. Thus, in these preferred embodiments, the process of the present invention provides a real-time, feedback control process for monitoring and/or producing chemical slurries.
The present invention also provides a chemical mixing system for monitoring and/or making a slurry having a desired solids content within a qualification range. Generally, the chemical mixing system comprises a mix volume adapted to receive at least two chemical components from at least first and second supply sources. Additionally, the system comprises a control system that is responsive to a conductivity measurement of the resulting combination as the components are added to the mix volume. More specifically, the control system is capable of generating a control signal in response to the conductivity measurement which, in turn, controls the addition of at least one of the chemical components to the mix volume. In this manner, when the combination of chemical components, i.e., the blended slurry, has a conductivity value corresponding to the reference conductivity value, the addition of the at least one chemical component is ceased.